In general, hydraulic turbines such as a Francis turbine, a Kaplan turbine or a Bulb turbine serving as an axial flow turbine or the like are known. Further, as a hydraulic turbine capable of conducting both power generation and pumped storage, a Francis pump hydraulic turbine is known. Here, the Francis pump hydraulic turbine (hereinafter simply referred to as Francis turbine) will be described by way of example.
In the Francis turbine, water flows from an upper reservoir into a spiral casing, and the water flowing into the casing flows into a runner through stay vanes and guide vanes. The guide vanes are configured to be rotated to change an opening degree, thereby enabling adjustment of a flow rate of the water flowing into the runner. The runner is rotated around a hydraulic turbine rotating axis by the water flowing thereinto. Thereby, power is generated at a generator connected to the runner via a main shaft. The water flowing out of the runner is discharged to a lower reservoir through a draft tube.
In this way, the water from the upper reservoir is discharged to the lower reservoir through the casing, the stay vanes, the guide vanes, the runner, and the draft tube. In the meantime, the water flows along a running water surface of a channel which is defined by the running water surface of, for instance, the casing. Thereby, a friction loss occurs at a flow of the water along the running water surface. The friction loss is different depending on a flow velocity and a Reynolds number. In general, the higher the flow velocity, the greater the friction loss, whereas the smaller the Reynolds number, the greater the friction loss.
In the Francis turbine, various losses in addition to the friction loss can occur. For example, a secondary flow loss caused by formation of a flow that does not follow a main flow, a separation loss caused by generation of separation from a flow, and a vortex loss caused by a swirl flow generated from a runner outlet in a draft tube are exemplified.
The secondary flow loss and the separation loss or the like can be reduced by optimizing a shape of each part. However, even in the case of optimizing the shape of each part, the friction loss occurs at a flow of water along a running water surface of each part. For this reason, there is a limitation in reducing the losses of the Francis turbine as a whole by reducing the secondary flow loss and the separation loss or the like.